High-frequency is here understood as an electromagnetic signal within a frequency range of approximately 1 kHz to circa 300 GHz; especially, or in a narrower sense, from 1 MHz to 10 GHz. Optical signals, however, are understood as electromagnetic signals within a frequency range of about 1 THz to 3000 THz. The latter frequency range does not only include visible light, but also infrared or ultraviolet light.
Such a system usually includes two sources of coherent light (laser) and an optical system for superimposing and filtering the two coherent light fields. The two sources of coherent radiation can each be continuous wave optical lasers or pulsed lasers or frequency combs. The continuous wave optical laser can be a gas, diode, fiber or solid state laser. The pulsed laser can be a gas, diode, fiber or solid state laser. The frequency comb can be generated by a short-pulse laser or by non-linear effects (e. g. microcombs according to EP 1 988 435 A1 or US 2008/0285606 A1) or by modulation of cw lasers or by optical rectification, difference frequency generation or other means.
A plurality of applications requires the stabilization of a laser onto a reference or the measurement of a laser against a reference. This reference can be an optical reference or a radio-frequency reference (i. e. high-frequency reference, such as an Rb- or Cs-atomic clock, an H-maser or a GPS-receiver). The optical reference can be another laser either at a nearly identical optical frequency or at a different optical frequency. In particular, the reference can be a laser locked to a stable high finesse cavity or to an optical transition of an atom, an ion or a molecule. In the first case, the two lasers are directly superimposed in a system and measured or stabilized. In the latter case, a frequency comb can bridge the difference of the two optical frequencies while preserving phase coherence. Here, two systems are advantageous, one for superimposing the optical reference with the frequency comb, and a second for superimposing the laser to be stabilized or measured with the frequency comb. In the stabilization or measurement against a radio-frequency reference, this serves to stabilize the frequency comb. The laser to be measured or stabilized is again superimposed with the frequency comb. Vice-versa, the frequency comb can be stabilized to an optical reference. For this, up to two systems are required for stabilizing the two degrees of freedom, i. e. repetition rate and offset frequency, of the frequency comb.
A system for generating a beat signal in the form of a high-frequency signal from two optical signals is described in DE 10 2004 037 549 A1. This system is designed for telecommunications engineering. The light of two pump lasers is irradiated into an optical wave guide in a direction opposite to that of the light of a signal laser. In the process, side bands are obviously formed which are supplied to a photodiode and measured there.
US 2003/0223757 A1 describes a system for generating an RF frequency standard (RF stands for radio frequency, i. e. high frequency). Here, the light of a cw laser is modulated to generate an optical frequency comb. In two separate paths, one frequency component each is filtered out from the frequency comb. By means of these two filtered-out frequency components, an RF beat signal is generated by superimposition on a detector.
JP 02257026 A and DE 196 33 428 A1 each disclose optical systems where light of a tunable light source is superimposed by measuring light of unknown frequency components. DE 196 33 428 A1 suggests a depolarizer for the tunable light. JP 02257026 A suggests using the high-frequency signal for measuring the frequency stability of a laser.
US 2006/0251424 A1 discloses a further arrangement for generating RF beat signals. Here, a frequency comb is generated, for example, in an optical fiber subsequently treated by generating a Bragg grating, and superimposed with the line of a further laser for generating a beat signal.
Frequency stabilization of frequency comb generators using high-frequency beat signals is described in DE 100 44 404 A1, DE 10 2005 035 173 A1, and DE 199 11 103 B4 as well as in EP 1 372 275 B1. As frequency comb generator, a short-pulse or ultrashort-pulse oscillator is provided there, i. e. a mode-coupled laser with pulse durations within a range of femto- (fs) to nanoseconds (ns). If one performs a Fourier transformation from the time domain to the frequency domain, a “frequency comb” corresponds to the series of laser pulses in the frequency domain. It is composed of a plurality of sharp, δ-like functions at different discrete frequencies, referred to as modes fn. Adjacent modes have a distance Δf from each other which exactly corresponds to the pulse repetition rate (=repetition rate) of the oscillator and which is therefore determined by the optical path length of the pulses in the oscillator.
However, the modes of the frequency comb are normally not exactly an integral multiple of Δf, but the complete frequency comb is shifted by a so-called offset frequency f0. Mathematically, the frequency comb can therefore be described as fn=f0+nΔf. The origin of the offset frequency f0 consists in the group velocity for the pulses circulating in the oscillator, which determines the repetition rate and thereby the mode distance Δf, differing from the phase velocity of the individual modes.
In DE 199 11 103 A1, EP 1 161 782 B1, and DE 100 44 404 C2, methods are described by which the two degrees of freedom of the frequency comb, i. e. the offset frequency f0 and the mode distance Δf, can be fixed or set to fixed values. To this end, one stabilizer or control loop each is provided. A first stabilizer relates to the mode distance. As a measured value for this stabilizer, the pulse repetition rate (optionally divided into or multiplied to ranges that can be better detected) can be used which corresponds—as illustrated—to the mode distance. An evaluation and comparison unit compares the measured value with a given reference value for the pulse repetition rate. To change the mode distance or to adjust it to the given reference value with the deviation being fixed, the stabilizer controls an actuator which changes the optical path length of the oscillator and thus the pulse repetition rate. For example, the actuator can be a linear drive or a piezo actuator for a resonant cavity mirror of the oscillator.
A second stabilizer controls the offset frequency f0 to a certain value. For this purpose, a certain mode fn of the frequency comb is superimposed on a detector (e. g. a photodiode or a photomultiplier) either with an external, exactly known reference frequency (e. g. from a continuous wave optical laser) or with a frequency-doubled mode from the same frequency comb. The superimposition generates a beat frequency in the radio-frequency range on the detector. An evaluation and comparison unit compares the beat frequency with a given, optionally variably adjustable reference frequency. If a deviation is detected, the second stabilizer controls an actuator which changes the difference between the phase and group delay time in the oscillator. This can be accomplished, for example, by slightly tipping an end resonant cavity mirror in a resonant cavity branch through which the modes pass separately to change the optical path length of the oscillator depending on frequency. As an alternative, the pumping power for the oscillator could be changed, or a dispersive element, such as a pair of prisms or a transparent tilting plate, could be inserted into the beam path of the oscillator and its position could be changed. Especially in a fiber laser any change of the optical power circulating inside the laser cavity will serve to adjust the offset frequency.
With the means described in DE 199 11 193 A1, EP 1 161 782 B1, or DE 100 44 404 C2, altogether a completely stabilized frequency comb is generated whose individual modes are at exactly known frequencies and coherent with respect to each other. In view of the detailed description of these means, reference is made to the three mentioned documents.
DE 10 2007 025 037 B3 discloses a method for determining a frequency and/or phase difference, DE 10 2008 062 139 A1 a method for providing a reference frequency from beat signals, DE 10 2004 022 037 A1 a method for generating a frequency comb with offset-free frequencies, EP 1 258 718 A1 a system for measuring a group velocity dispersion, and JP 06130247 A an optical assembly in which a frequency shifter in a ring fiber takes care that frequency-shifted light can be superimposed by the original light into a beat signal.
Moreover, methods for generating such a beat signal in a free-beam assembly are already known (Reichert et al., Optics Communications 172, 59-68 (1999)).